I work with computers as pieces of hardware extensively. I have done a great deal of programming, 3D modelling/rendering, drawing/image manipulation in Photoshop, gaming and other PC intensive tasks. I enjoy and keep up with PC building as a hobby. My brother, who has a computer science degree and does QA on datacenter equipment, recently asked ME to pick the parts for his new computer. I hope that establishes a degree of crebility here.
I would like very muuch to light both Apple and Microsoft on fire, but for 99.9% of people it makes much more sense to buy Windows machine. They run more programs(though this is not as true as it used ot be), you have many more options in terms of machines and they are cheaper. The only advantage I see in a Mac is that it will probably hold its value a bit better, but if you plan to run the computer until it dies then this doesn't matter.
The advice above about screens above I wouldn't argue with for a moment. In fact, you may wish to buy a cheaper laptop and a nice monitor since that will give you more options and will mean that you can keep the screen when you eventually change laptops.
(Sorry, didn't see this was already suggested)
As far as processors, don't sweat it that much. Because AMD's good laptop CPUs (Ryzen) were only recently brought to market, you will probably want to stick with Intel. I wouldn't get anything less than an i3, but the fact is that even that is fast enough to do most things tolerably and my experience has been that Photoshop isn't great at taking advantage of the additional threads offered by higher end chips. This said, I have seen i7s in pretty cheap laptops. Consider i3 a minimum and anything else a nice bonus that you will sometimes notice.
8 GB of RAM is minimum and basically ok, 12 would be nice, 16 is the outside edge of reasonableness. RAM can be upgraded later if you find you don't have enough, and RAM prices are quite high right now (in fact China is credibly investigating manufacturers for price fixing) so this will make a difference in the cost of the machine.
More HHD space is better, but you can always store the bulk of your photos on an external drive. Oh, and on that note GET AN EXTERNAL DRIVE FOR BACKUP! Hard drivers fail. You can use a program called freesync to easily back up everything from one drive to another.
You really want something with an SSD in it. Just having the OS on an SSD will make the computer infinitely less of a pain to use. Try to get something with at least a 64 GB SSD in it. If the SSD is bigger, you can pure more commonly used programs on it which will make those more responsive as well. An SSD is far more important than a faster processer once you get to the i3s. Without one your CPU will sit idle much of the time while it waits for things to load off the HDD.
If at all possible, get something with a dedicated graphics chip of some kind in it. This usually adds at least 100 dollars to the cost of the machine, but the improvement in smoothness of any graphics application is meaningful.
Don't buy an Acer or HP. That's just experience talking.
I hope that is useful, I would be happy to help in any other way I can. If it were me I would go the seperate monitor route, but I have a desktop with 3 monitors so I'm probably a slightly different kind of user
